The project is designed to provide a detailed description of the brain of the horseshoe crab with emphasis on the optic centers, their interconnections and synaptic architecture. Subjects to be investigated include the ganglia of the compound eye, i.e., lamina and medulla; ganglia of the median eye; the central body, an integration area for all visual inputs; and the corpora pedunculata, a massive lobe of the brain engaged in the processing of all sensory modalities. Specific problems relate to synaptic contacts of eccentric and retinula cells in the lamina; functional morphology of neuroglial cells in the central nervous system; and the quantitation of fiber tracts to the corpora pedunculata as it relates to synaptic divergence in the glomeruli and convergence in the neuropile of the peduncles. The study will utilize light microscopic methods of serial sectioning, reconstructions, and scanning electron microscopy for general anatomical orientation, but primarily electron microscopy in the analysis of neural networks and synaptic regions, buttressed by Golgi impregnations, selective staining techniques, and degeneration methods to enhance the specificity of identifications. Functional interpretations will take cognizance of the wide spectrum of available physiological studies on the visual system and should make this brain an attractive object for neurophysiological exploration.